In the oil and gas industry, seismic exploration techniques are commonly used to aid in locating subsurface deposits of hydrocarbons and other useful minerals. Seismic exploration, whether on land or at sea, is a method of detecting geologic structures below the surface of the earth by analyzing seismic energy that has interacted with the geologic structures. Generally, a seismic vibratory source imparts a force at the surface of the earth. The resulting mechanical stress propagates according to the elastic properties of the subsurface, and is at least partially reflected by subsurface seismic reflectors (interfaces between geologic structures that have different acoustic impedances). Seismic receivers, placed at or near the earth's surface, within bodies of water, or below the earth's surface in wellbores, record the ground motion or fluid pressure resulting from the reflection. The recordings are processed to generate information about the location and physical properties of the subsurface geologic structures that reflected the seismic energy.
Various types of sources have been employed to impart seismic energy into the earth, but most fall into one of two general categories: impulsive or vibratory. An impulsive source, such as an explosive or airgun, generates a short, high-amplitude force, injecting a large amount of energy into the ground in a brief time. Recordings generated using impulsive sources generally have a high signal-to-noise ratio, which facilitates subsequent processing. However, the use of impulsive sources can present safety or environmental concerns.
By contrast, a vibratory seismic vibratory source generates a lower-amplitude force over a longer period of time. The resulting recordings generally have a lower signal-to-noise ratio than those generated with impulsive sources. Furthermore, because the imparted force typically extends over a time much longer than the interval between reflections, the recorded data generally contains multiple overlapping reflections. However, subsequent processing can correlate the recorded data with the sweep reference signal to collapse the data to produce a correlated shot gather that resembles a shot gather acquired using an impulsive source.
Vibratory sources can take a number of different forms. For example, recent land surveys have often employed servo-controlled hydraulic “shaker units” mounted on trucks. Marine vibratory sources used in the recent past include a towed bell-shaped housing, with an acoustic piston in its open end driven by a hydraulic system similar to the land-based shaker units. However, alternative designs have been used, and the term “vibratory source” is intended to encompass any seismic vibrator, whether used on dry land or at sea.
Vibratory sources also permit some control over the characteristics of the imparted force. For example, to facilitate data collection, subsequent data processing, or both, it is often desirable to impart a force with energy at one or more desired frequencies, and to vary those frequencies over time. Such a controlled force is typically referred to as a “sweep.” The difference between the highest and lowest frequencies contained in the sweep is known as the “frequency range” of the sweep, and the length of time over which the source generates the sweep is known as the “sweep time.” Many different forms of sweep may be useful in a seismic survey. For example, a sweep may include a single sinusoid at a fundamental frequency that starts low and varies monotonically upward (an “upsweep”) or a fundamental frequency that starts high and varies monotonically downward (a “downsweep”). Such sweeps may be linear, with the fundamental frequency changing at a fixed rate over the entire sweep time. Sweeps may also be nonlinear, for example a quadratic or logarithmic sweep. Alternatively, a sweep may include an unvarying fundamental frequency, a mixture of multiple frequencies, an exotic signal such as a pseudo-random sequence, or any other desired signal.
Vibratory sources are generally controlled by a sweep generator. In most cases, the sweep generator outputs a pilot signal that is the same as the sweep reference signal, which is delivered to the source controller as an input. The source controller in combination with the vibrator actuator operates as a closed-loop feedback control system. Ideally, the source would generate a duplicate of the pilot signal as the force imparted into the earth. However, for a variety of reasons, the force actually generated by a vibratory source may differ from the pilot signal and consequently may differ from the sweep reference signal that is used as the correlation operator. For example, machinery associated with the source may introduce noise—undesired energy not present in the pilot signal. Noise may also result from characteristics of the earth, such as the mechanical impedance of the ground, from variations in the location, bad coupling due to operation on uneven surfaces or orientation of the source, or from other factors. Even though the vibrator control is a closed-loop feedback system, the closed loop bandwidth of the controllers in common use today may be insufficient to mitigate this noise. To facilitate subsequent data processing, and thus ultimately to aid in the detection of subsurface geologic structures, it is desirable to reduce or eliminate noise introduced into the sweep generated by a seismic vibratory source.